Specific structures on silicon-on-insulator (SOI) substrates tend to have problems with heat accumulation from self-heating due to the very low thermal conductivity of the SOI substrate. This presents particular issues with the maximum allowed current density of thermally sensitive structures. The heat accumulation presents particular problems with DC structures such as, for example, some precision resistors (e.g., thin film resistors).
Precision resistors are in general use in Si-based microelectronics integrated circuit chips. These resistors are frequently fabricated from polysilicon layers deposited on the chip, but they can also be made from diffused silicon (Si) layers in SOI wafers. These resistors produce heat when current flows through them. In particular, polysilicon and diffused resistors, especially those formed on SOI wafers, heat up rapidly with increasing current density. Although the resistor itself can tolerate relatively high temperatures without suffering damage, wiring on the various metallization levels above and nearby the resistors becomes much more vulnerable to failure by electromigration due to the heating caused by the resistor. Generally, a temperature increase of 5° C. in a metal line can decrease the lifetime of the line by 25 to 30%. The generated heat can also permanently alter the value of the resistance of the resistor by changing the grain size of the polysilicon, by burning out portions (or all) of the film and by redistributing the dopant atoms. Consequently, limiting the current through the resistor protects both the resistor stability and the integrity of the nearby metallization.
However, limiting the current through a resistor is at odds with the continued drive toward circuit miniaturization and the trend toward progressively greater current densities for high-performance circuits. The miniaturization of features typically involves reducing the film thickness in which resistors are formed, which tends to increase current density, which causes the resistor to generate more heat.
Accordingly, there exists a need in the art to overcome the deficiencies and limitations described hereinabove.